1. Field of the Invention
This invention relates generally to thermoplastic packaging materials and, more specifically, to flexible, multilayer films.
2. Background of the Invention
Many food products are processed in thermoplastic film packages by subjecting the packaged product to elevated temperatures produced by, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in, and films used in such processes are known as cook-in films.
A food product that is packaged and processed in this manner can be refrigerated, shipped, and stored until the food product is to be consumed or further processed by, for example, slicing and repackaging into smaller portions for retail display. Alternatively, the processed food can be removed immediately from the cook-in package for consumption or further processing (e.g., sliced and repackaged).
A cook-in film must be capable of withstanding exposure to rather severe temperature conditions for extended periods of time while not compromising its ability to contain the food product. Cook-in processes typically involve a long cook cycle. Submersion in hot (i.e., about 55xc2x0 to 65xc2x0 C.) water for up to about 4 hours is common; submersion in 70xc2x0 to 100xc2x0 C. water or exposure to steam for up to 12 hours is not uncommon, although most cook-in procedures normally do not involve temperatures in excess of about 90xc2x0 C. During such extended periods of time at elevated temperatures, any seams in a package formed from a cook-in film preferably resist failure (i.e., pulling apart).
Following the cook-in process, the film or package preferably conforms, if not completely then at least substantially, to the shape of the contained food product. Often, this is achieved by allowing the film to heat shrink under cook-in conditions so as to form a tightly fitting package. In other words, the cook-in film desirably possesses sufficient shrink energy such that the amount of thermal energy used to cook the food product also is adequate to shrink the packaging film snugly around the contained product. Alternatively, the cook-in film package can be caused to shrink around the contained food product prior to initiating the cook-in procedure by, for example, placing the package in a heated environment prior to cooking.
Some cook-in applications impose some very stringent performance requirements on films for use therewith. For example, some food products that are processed via cook-in procedures are oxygen sensitive. Cook-in films for these products need to include one or more oxygen barrier layers. Other cook-in applications require that the film or the package made therefrom be printable and be able to retain any image printed thereon.
An increasingly important requirement of cook-in films is that they have good interply adhesion. This is complicated where a layer derived primarily from a homo- or co-polymer of propylene is to be adhered directly to a layer derived primarily from a homo- or co-polymer of ethylene. Although ethylene and propylene are homologues, polymers made from one tend not to adhere well to polymers made from the other. One attempt to counteract this tendency toward poor adhesion has involved blending a polymer including mer units derived from propylene and having a low Vicat softening point with a homo- or co-polymer of ethylene so as to increase the compatibility of the layer formed therefrom with the layer derived primarily from a homo- or co-polymer of propylene. However, even where such a film as made exhibits good interply adhesion, that same film can exhibit mediocre or even poor interply adhesion after it is oriented.
Further, when films of this type are sealed, the sealing process can induce delamination between the seal layer and the layer adjacent thereto. During the cooking process, these same two layers must provide the film with structural integrity and support the seal formed in the seal layer. Also, the seal layer needs to be able to resist the degenerative effects of grease and/or fatty products which often are encountered during cook-in processes. Thus, the need remains for an oriented film with good interply adhesion and sealability.
Briefly, the present invention provides an oriented multilayer film that includes at least two layers. One of these layers is an outer layer that includes a polymer including mer units derived from propylene. Directly adhered to this outer layer is a second layer including at least one member selected from the group consisting of ethylene/alkyl acrylate copolymer, propylene/(xcex1-olefin copolymer having a propylene mer content less than that of the polymer in the first layer and a Vicat softening point of at least 50xc2x0 C., and ethylene/vinyl acetate copolymer with at least 15 weight percent vinyl acetate mer content, wherein the ethylene/alkyl acrylate copolymer has a melting point at least about 3xc2x0 C. higher than a reference ethylene/alkyl acrylate copolymer, wherein said reference ethylene/alkyl acrylate copolymer has the same amount and type of alkyl acrylate and ethylene, and wherein said reference copolymer is made in a multi-zone autoclave reactor where the ratio of alkyl acrylate to ethylene in a reaction zone is about equal to the overall ethylene to alkyl acrylate ratio fed to the multi-zone autoclave reactor.
The film also can include one or more other layers such as, for example, bulk layers, O2-barrier layers, and/or abuse layers.
Articles made from the above-described film (e.g., bags and casings), methods of making the film, and methods of using the film also are provided.
Those of ordinary skill in the art recognize that polymers derived primarily from propylene and polymers derived primarily from ethylene tend not to adhere well to each other. Films including adjacent layers derived from these dissimilar materials can have less-than-optimal adhesion, orientability, degraded optics, and/or gauge non-uniformities, perhaps due to differential stresses within the layers. Nevertheless, work leading to the film of the present invention has shown that layers including at least one of the foregoing co-polymers have good bond strength with layers including a propylene homo- or co-polymer, that a film including such layers can have good optical characteristics, and that a film including such layers can have uniform gauge. In view of the fact that interply bond strength is known to decrease significantly as molecular orientation increases (as occurs when a film is oriented), the good interply adhesion exhibited by the oriented film of the present invention is even further surprising.
Furthermore, when the second layer includes a propylene/xcex1-olefin copolymer having a propylene mer content less than that of the first layer, good interply adhesion is exhibited with the polypropylene-containing first layer. That the propylene/xcex1-olefin copolymer of the second layer also exhibits good adhesion to the adjacent polyethylene layer is surprising. That a propylene/xcex1-olefin copolymer can maintain good interply adhesion with all adjacent layers, even while the film is undergoing orientation, also is surprising. Most surprising is that a second layer which includes a copolymer having a Vicat softening point significantly lower than the cook temperature can maintain good adhesion to adjacent layers through cooking procedures at temperatures up to about 93xc2x0 C.
To assist in understanding the more detailed description of the invention that follows, certain definitions are provided immediately below. These definitions apply herein throughout unless a contrary intention is explicitly indicated:
xe2x80x9cpolymerxe2x80x9d means the polymerization product of one or more monomers and is inclusive of homo-polymers as well as copolymers, terpolymers, tetrapolymers, etc., and blends and modifications of any of the foregoing;
xe2x80x9cmer unitxe2x80x9d means that portion of a polymer derived from a single reactant molecule; for example, a mer unit from ethylene has the general formula xe2x80x94CH2CH2xe2x80x94;
xe2x80x9chomopolymerxe2x80x9d means a polymer consisting essentially of a single type of repeating mer unit;
xe2x80x9ccopolymerxe2x80x9d means a polymer that includes mer units derived from at least two reactants (normally monomers) and is inclusive of random, block, segmented, graft, etc., copolymers, as well as copolymers, terpolymers, tetrapolymers, and the like;
xe2x80x9cpolyolefinxe2x80x9d means a polymer in which some mer units are derived from an olelinic monomer which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted (e.g., olefin homopolymers, copolymers of two or more olefins, copolymers of an olefin and a non-olefinic comonomer such as a vinyl monomer, and the like);
xe2x80x9c(meth)acrylic acidxe2x80x9d means acrylic acid and/or methacrylic acid;
xe2x80x9c(meth)acrylatexe2x80x9d means acrylate and/or methacrylate;
xe2x80x9canhydride-graftedxe2x80x9d means a group containing an anhydride moiety, such as that derived from maleic acid, fumaric acid, etc., has been chemically attached to or affiliated with a given polymer;
xe2x80x9cpermeancexe2x80x9d (in the packaging industry, xe2x80x9cpermeancexe2x80x9d often is referred to as xe2x80x9ctransmission ratexe2x80x9d) means the volume of a gas (e.g., O2) that passes through a given cross section of film (or layer of a film) at a particular temperature and relative humidity when measured according to a standard test such as, for example, ASTM D 1434 or D 3985;
xe2x80x9clongitudinal directionxe2x80x9d means that direction along the length of a film, i.e., in the direction of the film as it is formed during extrusion and/or coating;
xe2x80x9ctransverse directionxe2x80x9d means that direction across the film and perpendicular to the machine direction;
xe2x80x9cfree shrinkxe2x80x9d means the percent dimensional change, as measured by ASTM D 2732 (incorporated herein by reference), in a 10 cmxc3x9710 cm specimen of film when subjected to heat;
xe2x80x9cshrink tensionxe2x80x9d means the force per average cross-sectional area developed in a film, in a specified direction and at a specified elevated temperature, as the film attempts to shrink at that temperature while being restrained (measured in accordance with ASTM D 2838, which is incorporated herein by reference);
as a verb, xe2x80x9claminatexe2x80x9d means to affix or adhere (by means of, for example, adhesive bonding, pressure bonding, corona lamination, and the like) two or more separately made film articles to one another so as to form a multilayer structure; as a noun, xe2x80x9claminatexe2x80x9d means a product produced by the affixing or adhering just described;
xe2x80x9cdirectly adhered,xe2x80x9d as applied to film layers, means adhesion of the it subject film layer to the object film layer, without a tie layer, adhesive, or other layer therebetween;
xe2x80x9cbetween,xe2x80x9d as applied to film layers, means that the subject layer is disposed in the midst of two object layers, regardless of whether the subject layer is directly adhered to the object layers or whether the subject layer is separated from the object layers by one or more additional layers;
xe2x80x9cinner layerxe2x80x9d means a layer of a film having each of its principal surfaces directly adhered to one other layer of the film;
xe2x80x9couter layerxe2x80x9d means a layer of a film having less than both of its principal surfaces directly adhered to other layers of the film;
xe2x80x9cinside layerxe2x80x9d means the outer layer of a film in which a product is packaged that is closest, relative to the other layers of the film, to the packaged product;
xe2x80x9coutside layerxe2x80x9d means the outer layer of a film in which a product is packaged that is farthest, relative to the other layers of the film, from the packaged product;
xe2x80x9cbarrier layerxe2x80x9d means a film layer with a low permeance toward one or more gases (e.g., O2);
xe2x80x9cabuse layerxe2x80x9d means an outer layer and/or an inner layer that resists abrasion, puncture, and other potential causes of reduction of package integrity and/or appearance quality;
xe2x80x9ctie layerxe2x80x9d means an inner layer having the primary purpose of providing interlayer adhesion to adjacent layers that include otherwise non-adhering polymers;
xe2x80x9cbulk layerxe2x80x9d means any layer which has the purpose of increasing the abuse resistance, toughness, modulus, orientability, etc., of a multi-layer film and generally comprises polymers that are inexpensive relative to other polymers in the film;
xe2x80x9cseal layerxe2x80x9d (or xe2x80x9csealing layerxe2x80x9d or xe2x80x9cheat seal layerxe2x80x9d or xe2x80x9csealant layerxe2x80x9d) means the outer layer(s) involved in the sealing of the film to itself, another layer of the same or another film, and/or another article which is not a film and
(a) with respect to packages with fin seals, the phrase generally refers to the inside layer, which frequently also serves as a food-contact layer in the packaging of foods (although, in a multi-layer film, the composition of the other layers within about 0.075 mm of the surface also can affect sealability and seal strength), or
(b) with respect to packages with lap seals, the phrase generally refers to both the inside and outside layers of the film.
as a noun, xe2x80x9csealxe2x80x9d means a bond of a first region of a film surface to a second region of a film surface (or opposing film surfaces) created by heating (e.g., by means of a heated bar, hot wire, hot air, infrared radiation, ultrasonic sealing, etc.) the regions (or surfaces) to at least their respective softening points;
xe2x80x9ccorona treatmentxe2x80x9d or xe2x80x9ccorona discharge treatmentxe2x80x9d means a process in which one or both primary surfaces of a thermoplastic film are subjected to the ionization product of a gas (e.g., air) in close proximity with the film surface(s) so as to cause oxidation and/or other changes to the film surface(s); and
xe2x80x9ccookxe2x80x9d means to heat a food product thereby effecting a change in one or more of the physical or chemical properties thereof (e.g., color, texture, taste, and the like).
Some films, including many which are used in cook-in processes, are oriented prior to use. Orientation involves stretching a film at an elevated temperature (the orientation temperature) followed by setting the film in the stretched configuration (e.g., by cooling). When an unrestrained, non-annealed, oriented polymeric film subsequently is heated to its orientation temperature, heat shrinkage occurs and the film returns almost completely to its original, i.e., pre-oriented, dimensions.
An oriented film has an orientation ratio, which is the multiplication product of the extent to which the film has been expanded in several directions, usually two directions perpendicular to one another. Expansion in the longitudinal direction, sometimes referred to as the machine direction, occurs in the direction the film is formed during extrusion and/or coating. Expansion in the transverse direction means expansion across the width of the film and is perpendicular to the longitudinal direction.